When a molecular ion generated from a sample molecule is made to move in a gas medium (or liquid medium) under the effect of an electric field, the ion moves at a speed proportional to the mobility which is determined by the strength of the electric field, size of the molecule and other factors. Ion mobility spectrometry (IMS) is a measurement technique which utilizes this mobility for an analysis of sample molecules. FIG. 8A is a schematic configuration diagram of a commonly known ion mobility analyzer (for example, see Patent Literature 1).
This ion mobility analyzer includes an ion source 1 for ionizing component molecules in a sample, a drift region 4 for measuring the ion mobility provided within a housing (not shown) which, for example, has a cylindrical shape, and a detector 5 for detecting ions which have traveled through the drift region 4. A shutter gate 3 is provided at the entrance end of the drift region 4, in order to send the ions produced by the ion source 1 into the drift region 4 in a pulsed form confined to an extremely narrow width of time. The inside of the housing is maintained at an atmospheric pressure or a low-vacuum state of approximately 100 Pa. Within the drift region 4, a uniform electric field having a downward potential gradient in the moving direction of the ions (to accelerate the ions) is created by the DC voltages respectively applied to a large number of annular electrodes included in the drift electrode group 2 arrayed within the drift region 4. FIG. 8B is a schematic diagram showing the potential distribution on the ion beam axis C within the drift region 4. A stream of neutral diffusion gas is formed against the direction of the acceleration by this electric field.
The ions generated in the ion source 1 are temporarily stopped at the shutter gate 3 provided at the entrance end of the drift region 4. When the shutter gate 3 is opened in a pulse-like manner, the ions in the form of a packet are introduced into the drift region 4. The introduced ions travel along the downward potential gradient, colliding with the diffusion gas within the drift region 4. During this process, the ions are temporally separated according to the ion mobility which depends on the size, three-dimensional structure, charge and other properties of the ions. Consequently, ions having different ion mobilities reach the detector 5 with certain intervals of time. If the electric field within the drift region 4 is uniform, the collision cross-section between each ion and the diffusion gas can be estimated from the drift time required for that ion to pass through the drift region 4.
In Patent Literature 2, a method is proposed in which, instead of creating the uniform electric field within the drift region, the potential distribution on the ion beam axis is regulated so that the ions are focused in the radial direction of the ion path. In this case, although it is considerably difficult to estimate the collision cross-section between the ion and the diffusion gas, the loss of the ions due to the diffusion is reduced, so that a greater amount of ions can reach the detector. Consequently, a higher level of sensitivity can be achieved.
In general, the resolving power of an ion mobility analyzer is defined as [drift time]/[peak width] on a spectrum with the horizontal axis representing the drift time and the vertical axis representing the signal intensity, as shown in FIG. 8C. Accordingly, in order to improve the resolving power, it is necessary to devise a technique for increasing the drift time or decreasing the peak width. To increase the drift time, it is necessary to elongate the drift region. This approach is considerably restricted in terms of the size and cost of the device. Furthermore, due to the diffusion caused by the collision with the diffusion gas, the packet of ions becomes more dispersed as they fly a longer distance, so that the peak becomes broader as the drift time becomes longer. Therefore, even if the drift region is elongated in order to increase the drift time, there will be only a minor improvement in the performance because the peak width also simultaneously increases. In summary, elongating the drift region to improve the resolving power is not so beneficial because this method has the significant demerits of increasing the cost and size of the device while providing only a minor improvement in the performance.